Left ventricular myotomy and myectomy has been shown as an effective operation for selected patients with IHSS. With increasing patient referrals, more centers are performing the operation, and it may prove helpful to obtain exact measurements of septal thickness during the myectomy. Theoretically, a servosystem could be designed, which would allow an exact depth to the myectomy, yet reliably avoid septal perforation and the creation of a ventricular septal defect. In this study septal thickness is measured by a specially designed echo probe which is inserted into the left ventricular outflow tract at the time of operation (myotomy and myectomy) for IHSS.